Don't reset completed status when completed and then value changed

mentoring/mentoring.py

@@ -119,6 +119,10 @@ class MentoringBlock(XBlockWithLightChildren):
                 child.save()
                 completed = completed and child_result['completed']
 
+        # Once it has been completed once, keep completion even if user changes values
+        if self.completed:
+            completed = True
+
         if completed:
             message = self.get_message_html('completed')
         else: